In modern society, electronic equipment serves a wide variety of purposes. Many examples of such electronic equipment allow an audio signal to be reproduced or received and output to an amplifier or speaker. The audio signal is then transduced so as to be audible to a listener. Examples of electronic audio equipment that reproduce audio signals include compact disc players, video cassette recorders (VCRs), and computers. Examples of electronic audio equipment that receive transmitted audio signals include televisions and radios.
As will be familiar to anyone who has used any of these electronic audio devices, the speaker or speakers of the device often produce an audible pop that is not part of a reproduced or received audio signal. This audio pop may be produced when power is initially supplied to the electronic audio device, when the power to the electronic audio device is turned off or when a device that can receive several different audio signals or channels is switched from one channel to another.
A conventional electronic audio device is illustrated in FIG. 1. FIG. 1 shows an electronic audio device 101 which can provide any one of three input audio signals; Audio 1, Audio 2, and Audio 3, as the output audio signal. A switch 102 connects a line driver 103, e.g. an amplifier, to any one of the three audio signals.
The line driver 103 is powered, for example, by an electric plug 106 which is plugged into an electrical wall outlet. Alternatively, the line driver may be powered by batteries. A switch 104 is provided between the power source 106 and the line driver 103. This switch 104 allows power to be selectively provided to the line driver 103 and is, typically, the ON/OFF switch for the electronic audio device 101.
When the line driver 103 is powered, the audio signal selected by the switch 102 is provided, through a capacitor 105, to an audio output unit 108. The audio output unit may be, for example, an amplifier and speaker incorporated into a television, a stereo, a recording system, a radio, a computer or any other electronic audio equipment.
Associated with each audio signal is a DC voltage level to which the capacitor 105 becomes charged. When this DC voltage level is changed significantly and abruptly, for example, by switching to another audio signal having a different DC voltage level, the change in the voltage level of the system is propagated through the system to the audio output unit 108. This sudden change in voltage level, when it arrives at the audio output unit 108, causes any speaker associated with the audio output unit 108 to emit an audio pop.
In addition to switching between audio signals, the DC voltage level of the system is also abruptly changed when the switch 104 is turned on or off. Thus when the electronic audio device 101 is switched on or off with the switch 104, the change in the DC level of the system may cause the audio output unit 108 to pop.
Audio pop is unpleasant to the listener and, over time, may cause damage or wear to the audio output unit 108. Accordingly, there is a need in the art for a method and circuit for preventing audio pop in electronic audio equipment.